But the signing of the law, which goes into effect Oct. 1, isn’t the end of the fight. A new battle begins to convince judges and voters that it was the right response to last year’s school shootings in Newtown, Conn.

The National Rifle Association on Wednesday renewed its promise to file a lawsuit challenging the constitutionality of the law. It bans the sale of nearly all semi-automatic rifles, plus magazines that hold more than 10 bullets, and requires new gun buyers to submit digital fingerprints to state police.

Supporters won’t rest, either. With federal gun-control legislation stalled in Congress, they will go on the offensive in coming days with a television ad campaign designed to build public support for the new law.

Ahead of a 2014 election in which every member of Maryland’s General Assembly will be up for reelection, the ad is designed to blunt criticism from would-be contenders hoping to use the legislation as a campaign issue. It’s also a way, proponents said, to demonstrate to lawmakers in less-blue states as well as in Congress that a vote for gun control can be a winning one.

“We want Marylanders to know how well this law will work,” said Vincent DeMarco, president of Marylanders to Prevent Gun Violence, the group that will begin airing the ad. “That way, they will strongly support it when the other side tries to defeat candidates who supported it.”

It’s a far different dynamic than that at play in the U.S. Senate. New York Mayor Michael R. Bloomberg (I) is leading a multimillion-dollar campaign mostly to unseat Democrats who opposed federal gun-control legislation.

“The big difference is we have won in Maryland, we haven’t won yet nationally,” DeMarco said. “Our job is to stay strong, to defend it — that’s a very different task than trying to change votes to pass it.”

Sen. James Brochin (D-Baltimore County) is the kind of lawmaker whom gun-control proponents hope to help. His newly redrawn district extends from north of Baltimore to the rural border of central Pennsylvania.

There, on Wednesday, he got an earful, again, from another potential constituent opposed to his gun vote, he said.

“Everywhere I go, at least one person goes into a 20-minute tirade,” Brochin said, adding that he blames gun-rights groups for spreading false information about the bill.

“I think the vast majority of people in the state don’t understand that if you currently own a gun — even several assault weapons — you get to keep them and do whatever you want with them,” he said.

Brochin, who has begun knocking on doors to ask for support but has not yet decided whether to run, said he hoped an ad campaign would educate the public. He said he’s not convinced, however, that it will turn the tide in his new district, which he estimates had a 38 percent Democratic turnout last election.

Statewide, a Washington Post poll in February found that Maryland residents overwhelmingly supported the licensing plan proposed by O’Malley (D). Fully 85 percent backed the plan, and 73 percent said they did so “strongly.”

It aims to curtail “straw purchases,” when one person buys a gun for another who would not pass a background check to do so. Under Maryland’s law, those seeking to buy any gun other than a hunting rifle or shotgun will need to first obtain a license, a process that will include submitting fingerprints, passing classroom and firing-range training, and undergoing more extensive background checks.

In the poll, Marylanders also supported banning high-capacity ammunition clips and assault weapons. They also supported an NRA proposal to put armed guards in every school in the state, a measure Maryland Democrats killed.

For DeMarco and his lobbyist, former O’Malley communications director Rick Abbruzzese, the ad campaign marks a public turn from the mostly behind-the-scenes efforts the two led to help pass the bill.

Even in the Democratic-controlled legislature, the fight was a tough one. At one point, they took Baltimore County Police Chief James Johnson to meet House Judiciary Chairman Joseph F. Vallario Jr. (Prince George’s). Johnson, one of President Obama’s advisers on gun control, pulled out an active evidence folder with a surveillance picture of a suspect holding an assault weapon pointed at a bank teller.

After more than a month with the bill, Vallario’s committee relented and passed the bill.

“I’ve been waiting 25 years for this,” DeMarco said of the governor’s bill signing, scheduled for Thursday morning.

On Wednesday, Alexa Fritts, spokeswoman for the NRA’s Institute for Legislative Action, issued a brief e-mail statement: “The National Rifle Association’s position and concerns will be made very clear when we file our lawsuit.”

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Aaron Davis is an investigative reporter who joined The Post as a staff writer in 2008. He has covered local, state and federal government, as well as the aviation industry and law enforcement, among other topics. He was a member of the Post team that was a finalist for the Pulitzer Prize for coverage of the 2013 Navy Yard shooting.